1. Field of the Invention
The present invention relates to an information processing apparatus utilizing the principle of a scanning tunneling microscope (hereinafter abbreviated as "STM"), and an electrode substrate and an information recording medium used in the apparatus.
2. Description of the Related Art
Nowadays, memory materials are being used in computers and apparatuses related thereto, video discs, digital audio discs, etc., equipment constituting the nucleus of the electronics industry. The development of new materials in this field is being very actively promoted. The characteristics required of memory material vary in accordance with the use for which it is intended. The following are characteristics generally required of memory material:
1 High density and large recording capacity;
2 High response speed in recording and reproduction;
3 Low power consumption; and
4 High productivity and low price.
Conventionally, semiconductor memories and magnetic memories made of semiconductors and magnetic substances have been the mainstream of memories for information processing. However, as a result of recent developments in laser technology, inexpensive high-density recording mediums have appeared which consist of optical memories using films of organic dyes, photopolymers, or the like.
Apart from this, an STM (a scanning tunneling microscope) has recently been developed which makes it possible to directly observe the electronic structure of a surface atom of a conductor (G. Binning et al., Phys. Rev. Let., 49,57 (1982)). With this microscope, it is possible to perform high-resolution measurement on a real image in space, whether it is monocrystalline or amorphous. Further, it has an advantage that it allows the specimen to be observed with low power without being damaged by electric current. Moreover, it can operate even in ambient atmosphere so that it can be used with respect to various types of materials. Thus, a wide range of applications are expected from the STM.
The STM utilizes the fact that a tunnel current flows when a metal probe (a probe electrode) is brought near a conductive substance, up to a distance of approximately 1 nm, while applying a voltage between them. This current is very sensitive to changes in the distance between the metal probe and the conductive substance, and even allows for the reading of a variety of information regarding the entire electron cloud in an actual space by performing scanning with the probe in such a way as to maintain the tunnel current constant. In that case, the resolution in the in-plane dimension is approximately 0.1 nm.
Thus, by utilizing the principle of the STM, there is a good possibility that a high-density recording/reproduction of the atom order (sub-nanometer) can be performed. For example, in the information processing apparatus disclosed in Japanese Patent Laid-Open Publication No. 61-80536, an electron beam or the like is used to write data onto a recording medium by removing atom particles adhering to the medium surface, and reproducing the data by means of an STM. According to the disclosure in the specification of U.S. Pat. No. 4,575,822, the tunnel current flowing between the recording medium surface and the probe electrode is used to effect recording by injecting electric charges into a dielectric layer formed on the medium surface. There has also been a method proposed in which a laser beam, electron beam, corpuscular beam or the like is used to perform recording by utilizing physical or magnetic disruption of the medium surface.
According to another proposed method, a material having a memory effect with respect to voltage/current switching characteristics, for example, a thin-film layer of a .pi.-electron-type organic compound, chalcogen compound or the like, is used as the recording layer to perform recording and reproduction with an STM (Japanese Patent Laid-Open Publication Nos. 63-161552 and 63-161553). Assuming that the recording bit size is 10 nm, this method makes it possible to perform recording and reproduction by as much as 10.sup.12 bit/cm.sup.2.
FIG. 8 shows a sectional view of a conventional recording medium together with the tip of a probe electrode 202.
Numeral 101 indicates a substrate; numeral 102, an electrode layer; numeral 103, a recording layer; numeral 104, a track; numeral 202, the probe electrode; numeral 401, a data bit recorded on the recording layer 103; and numeral 402, crystal grains generated during the formation of the electrode layer 102. Assuming that the electrode layer 102 is formed by a usual method, such as vacuum evaporation or sputtering, the size of the crystal grains 402 ranges from approximately 30 to 50 nm.
The distance between the probe electrode 202 and the recording layer 103 can be kept constant through a conventionally well-known circuit construction. That is, a tunnel current flowing between the probe electrode and the recording layer is detected, and its value is transmitted through a logarithmic compressor 302 and a low-pass filter 303 and then compared with a reference voltage. A Z-axis actuator 204 supporting the probe electrode is controlled in such a way that this comparison value approaches zero, thereby maintaining a constant distance between the probe electrode and the recording layer.
Further, by driving an XY-stage 201, the surface of the recording medium is traced by the probe electrode 202, and the high frequency component of a signal at an arbitrary point P is separated, thereby making it possible to detect the data of the recording layer 103. FIG. 9 shows a signal strength spectrum with respect to the signal frequency at point P at this time.
Any signals of a frequency component not higher than f.sub.0 are due to a gentle rise and fall of the substrate 101 caused by warp, distortion or the like. The signals around f.sub.1 are due to surface irregularities of the electrode layer 103, caused mainly by the crystal grains 402 generated during the formation of the material into an electrode. Symbol f.sub.2 indicates a recording data carrier component, and numeral 403 indicates a data signal band as shown in FIG. 6. Symbol f.sub.3 indicates a signal component generated from the atomic/molecular arrangement of the recording layer 103. Symbol f.sub.T indicates a tracking signal, which enables the probe electrode 202 to trace data arrays. It can be realized by forming a groove on the medium or writing thereto a signal which enables detection whenever an off-track condition occurs.
Use of a conventional recording medium based on an electrode substrate as described above entails the following problems:
1 To make use of the high resolution, which features the STM, and perform high-density recording, the data frequency component 403 must be between f.sub.1 and f.sub.3. For this purpose, a high-pass filter of a cut-off frequency of f.sub.c is used for data component separation. However, as shown in FIG. 9, a foot portion of the f.sub.1 signal component overlaps the data band 403. This is attributable to the fact that the f.sub.1 signal component is due to the crystal grains 402 of the electrode layer 102, with the recording size and bit interval of the data being in the range of 1 to 10 nm, which is close to the crystal grain size of 30 to 50 nm. As a result, the S/N ratio in data reproduction is deteriorated, thereby increasing the incidence of error.
2 The tracking signal f.sub.T can only be placed in the vicinity of f.sub.0. As a result, the frequency of the tracking signal is considerably low as compared with the data frequency, resulting in poor data tracing accuracy in tracking. This causes the incidence of error in data reading to be increased, thereby deteriorating the reliability of the information processing apparatus.
3 Further, a track groove corresponding to such a low tracking frequency is considerably large as compared to the data bit size, with the result that the data recording density is significantly low, thereby making it impossible to fully make use of the high resolution featuring the STM.